PMOS-output LDO with full spectrum PSR

ABSTRACT

A PMOS-output LDO with full spectrum PSR is disclosed. In one implementation, a LDO includes a pass transistor (MO) having a source coupled to an input voltage (Vin); a noise cancelling transistor (MD) having a source coupled to the Vin, a gate coupled to a drain and a gate of the pass transistor; a source follower transistor (MSF) having a source coupled to a drain of the pass transistor, a drain coupled to the drain and gate of the noise cancelling transistor; a current sink coupled between the drain of the source follower transistor and ground; and an error amplifier having an output to drive the gate of the source follower transistor.

FIELD OF DISCLOSURE

Aspects of the present disclosure relate generally to semiconductorcircuits, and more particularly to low dropout regulator (LDO) with fullspectrum power supply rejection (PSR).

BACKGROUND

Today, in many Systems on a Chip (SoC's), one or more regulated voltagesare used to power various subsystems within a SoC. Various types ofvoltage regulators have been developed to supply regulated voltages todifferent subsystems within the SoC. In general, a voltage regulatorgenerates a regulated output voltage from an input voltage. In low powerapplications, voltage regulators are often used to generate a lowerregulated output voltage than the input voltage. One common type of suchvoltage regulator is a low dropout regulator (LDO). LDOs can generallybe classified into two types, namely, n-type output LDOs and p-typeoutput LDOs.

FIG. 1 shows a conventional n-type output LDO 100. LDO 100 includes anerror amplifier 110, a pass transistor 120, a feedback resistor 130, anda resistor 140. Error amplifier 110 has a positive input terminal, anegative input terminal, an output, and a voltage supply input terminal.The voltage supply input terminal is coupled to a voltage supply Vdd,which can be at about 1.2V in some examples. The positive input terminalis configured to receive a reference voltage Vref. The negative inputterminal is coupled to one end of feedback resistor 130 and one end ofresistor 140. The other end of resistor 140 is coupled to ground. Theoutput of error amplifier 110 is coupled to a gate of pass transistor120. Pass transistor 120 is an n-type metal oxide semiconductortransistor (NMOS). Thus, LDO 100 is also referred to as an NMOS-outputLDO. The drain of pass transistor 120 is configured to receive an inputvoltage Vin, which can be about 0.85V in some examples. Note that Vdd ishigher than Vin typically for the conventional n-type output LDO 100. Asource of pass transistor 120 is configured to produce an output voltageVout of LDO 100. The source is also coupled to the other end of feedbackresistor 130 so that Vout is feedback to error amplifier 110 via thefeedback resistor 130. In other words, error amplifier 110, passtransistor 120, and feedback resistor 130 form a feedback loop withinLDO 100. The output voltage Vout is provided to a load, which isrepresented by a load capacitor 150 and a current source 160, which arecoupled to each other in parallel to the ground.

One important parameter that is often used to evaluate LDO's is powersupply ripple rejection ratio (PSRR). It is a measure of the outputripple compared to the input ripple over a wide frequency range (e.g.,10 Hz to 10 MHz) and is expressed in decibels (dB). One common way tocalculate PSRR for an LDO is:PSRR=20 log(A_(v)/A_(vo)),where A_(v) is the open-loop gain of the LDO feedback loop and A_(vo) isthe gain from Vin to Vout with the LDO feedback loop open. Thus, PSRRmeasures how well the LDO rejects ripple coming from the input powersupply at different frequencies.

Although the conventional n-type output LDO 100 can provide good PSRRand faster transient response, LDO 100 requires an additional highervoltage supply, namely, Vdd, to bias error amplifier 110. The other typeof conventional LDO, p-type output LDO, does not require an additionalhigher voltage supply, and one example of such a LDO is discussed below.

FIG. 2 shows a conventional p-type output LDO 200. LDO 200 includes anerror amplifier 210, a pass transistor 220, a feedback resistor 230, anda resistor 240. Error amplifier 210 has a positive input terminal, anegative input terminal, an output, and a voltage supply input terminal.Error amplifier 210 has a positive input terminal, a negative inputterminal, an output, and a voltage supply input terminal. The voltagesupply input terminal is coupled to an input voltage Vin. The negativeinput terminal is coupled to one end of feedback resistor 230 and oneend of resistor 240. The other end of resistor 240 is coupled to ground.The output of error amplifier 210 is coupled to a gate of passtransistor 220. Pass transistor 220 is a p-type metal oxidesemiconductor transistor (PMOS). The source of pass transistor 220 isconfigured to receive the input voltage Vin. A drain of pass transistor220 is configured to produce an output voltage Vout of LDO 200. Thedrain is also coupled to the other end of feedback resistor 240. Theoutput voltage Vout is provided to a load, which is represented by aload capacitor 250 and a current source 260, which are coupled to eachother in parallel to the ground. The p-type output LDO 200 uses Vin tobias error amplifier 210, and hence, LDO 200 does not require anyvoltage supply in addition to Vin. However, LDO 200 suffers from poorPSRR at mid-range frequencies, and hence, typically requires a largerload capacitor 250 to reduce power supply rejection (PSR). The largerload capacitor 250 increases the size of LDO 200.

Accordingly, there is a need in the art to design a LDO that does notrequire any voltage supply in addition to the input voltage Vin, whileproviding better PSRR in all ranges of frequency.

SUMMARY OF THE DISCLOSURE

The following presents a simplified summary of one or more embodimentsin order to provide a basic understanding of such implementations. Thissummary is not an extensive overview of all contemplatedimplementations, and is intended to neither identify key or criticalelements of all implementations nor delineate the scope of any or allimplementations. Its sole purpose is to present some concepts of one ormore implementations in a simplified form as a prelude to the moredetailed description that is presented later.

In one implementation, a LDO includes a pass transistor (M_(O)) having asource, a drain, and a gate, the source coupled to an input voltage(Vin); a noise cancelling transistor (M_(D)) having a source, a drain,and a gate, the source coupled to the Vin, the gate coupled to the drainof the noise cancelling transistor and the gate of the pass transistor;a source follower transistor (M_(SF)) having a source, a drain, and agate, the source coupled to the drain of the pass transistor, the draincoupled to the drain and gate of the noise cancelling transistor; acurrent sink coupled between the drain of the source follower transistorand ground; an error amplifier having an output, a positive inputterminal, and a negative input terminal, the output coupled to the gateof the source follower transistor, the positive input terminalconfigured to receive a reference voltage (Vref); and a feedbackresistor coupled between the drain of the pass transistor and thenegative input terminal of the error amplifier.

In one implementation, the LDO further includes a load capacitor coupledbetween the drain of the pass transistor and ground. The load capacitorhas a capacitance of about 2 pF.

In some implementations, the source follower transistor, the feedbackresistor, and the error amplifier are configured into a first loop.Furthermore, the pass transistor, the noise canceling transistor, andthe source follower transistor can be configured into a second loop.Further, the first loop can be configured to be a slow loop and thesecond loop can be configured to be a fast loop.

In some implementations, the error amplifier comprises a pair of p-typetransistors configured as a current mirror, and sources of the pair ofp-type transistors are coupled to Vin.

In some implementations, the error amplifier further comprises a firstn-type transistor configured as the positive input terminal and a secondn-type transistor configured as the negative input terminal.

A method to provide a regulated low voltage from an input voltage (Vin)has been disclosed herein. In some implementations, the method includesreceiving the input voltage at a source of a pass transistor (M_(O));coupling a source of a noise cancelling transistor (M_(D)) to the sourceof the pass transistor, wherein the noise cancelling transistor isconfigured into a diode; coupling a source of a source followertransistor (M_(SF)) to the drain of the pass transistor, and coupling adrain of the source follower transistor to a drain and a gate of thenoise cancelling transistor; coupling a current sink between the drainof the source follower transistor and ground; receiving a referencevoltage (Vref) at a positive input terminal of the error amplifier;coupling a feedback resistor between the drain of the pass transistorand a negative input terminal of the error amplifier to provide afeedback voltage to the error amplifier; generating an output of theerror amplifier based on the feedback voltage and the reference voltage;and driving the gate of the source follower transistor with the outputof the error amplifier.

In some implementations, the method further includes coupling a loadcapacitor between the drain of the pass transistor and ground. The loadcapacitor has a capacitance of about 2 pF.

In some implementations, the method further includes configuring thesource follower transistor, the feedback resistor, and the erroramplifier into a first loop. In some implementations, the method furtherincludes configuring the pass transistor, the noise cancelingtransistor, and the source follower transistor into a second loop. Thefirst loop can be a slow loop and the second loop can be a fast loop.

In some implementations, the error amplifier comprises a pair of p-typetransistors configured as a current mirror, and sources of the pair ofp-type transistors are coupled to Vin. Moreover, the error amplifier canfurther include a first n-type transistor configured as the positiveinput terminal and a second n-type transistor configured as the negativeinput terminal.

An apparatus to provide a regulated low voltage from an input voltage(Vin) has been disclosed herein. In some implementations, the apparatusincludes means for providing a slow loop to define an output voltage;and means for providing a fast loop to improve transient response,wherein the fast loop includes means for noise cancelling to improvepower supply ripple rejection ratio (PSRR).

In some implementations, the slow loop includes a source followertransistor, a feedback resistor, and an error amplifier.

In some implementations, the fast loop includes a pass transistor, anoise canceling transistor, and the source follower transistor.

To the accomplishment of the foregoing and related ends, the one or moreimplementations include the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrative aspects ofthe one or more implementations. These aspects are indicative, however,of but a few of the various ways in which the principles of variousimplementations may be employed and the description implementations areintended to include all such aspects and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional n-type output LDO 100.

FIG. 2 shows a conventional p-type output LDO 200.

FIG. 3 shows one implementation of a p-type output LDO 300.

FIG. 4A shows a loop phase plot of LDO 300 in one study of the slow loopof LDO 300.

FIG. 4B shows a loop gain plot of LDO 300 in one study of the slow loopof LDO 300.

FIG. 4C shows another loop phase plot of LDO 300 in one study of thefast loop of LDO 300.

FIG. 4D shows another loop gain plot of LDO 300 in one study of the fastloop of LDO 300.

FIG. 4E shows a PSRR plot of one implementation of LDO 300.

FIG. 5 shows a flow diagram of one implementation of a method to providea regulated low voltage from an input voltage (Vin).

DETAILED DESCRIPTION

The detailed description set forth below, in connection with theappended drawings, is intended as a description of variousconfigurations and is not intended to represent the only configurationsin which the concepts described herein may be practiced. The detaileddescription includes specific details for the purpose of providing athorough understanding of the various concepts. However, it will beapparent to those skilled in the art that these concepts may bepracticed without these specific details. In some instances, well-knownstructures and components are shown in block diagram form in order toavoid obscuring such concepts.

In order to provide a LDO with good PSRR in all ranges of frequencywithout using any voltage supply in addition to the input voltage (Vin),a fast loop and a slow loop are configured within a PMOS output LDOaccording to one aspect of the disclosure. The slow loop may include anerror amplifier, a source follower transistor, and a feedback resistorto define an output of the LDO. The fast loop may include the sourcefollower transistor, a pass transistor, and a noise cancellingtransistor. Both the noise cancelling transistor and the pass transistorare connected together at their gates. The sources of both the noisecancelling transistor and the pass transistor are configured to receiveVin. Further, the noise cancelling transistor is diode-connected, andhence, the noise cancelling transistor can effectively cancel noise inVin by putting the noise in Vin onto the gate of pass transistor. Insome implementations, the drain of the noise cancelling transistor isdirectly coupled to the drain of the source follower transistor. Bydirectly connecting the drains of the source follow transistor and thenoise cancelling transistor, the fast loop can be made more stablebecause there is only one low frequency pole. In some implementations,the drains of the source follow transistor and the noise cancellingtransistor is further coupled to a current sink. When load currentincreases significantly (e.g., from 4 mA to 10 mA), the current sink cansink more current momentarily until the slow loop can catch up. Theseand other advantageous features may be better appreciated through thefollowing detailed description of various implementations of the LDO.

FIG. 3 shows one implementation of a p-type output LDO 300. LDO 300includes an error amplifier 310, a pass transistor M_(O) 320, a feedbackresistor 330, a resistor 340, a load capacitor 350, a output currentsink 360, a noise cancelling transistor (M_(D)) 370, a source followertransistor (M_(SF)) 380, and a current sink 390. Error amplifier 310 hasa positive input terminal, a negative input terminal, an output, and avoltage supply input terminal. Error amplifier 310 has a positive inputterminal 310 a, a negative input terminal 310 b, an output 310 c, and avoltage supply input terminal 310 d. The voltage supply input terminal310 d is coupled to an input voltage Vin. The negative input terminal310 b is coupled to one end of feedback resistor 330 and one end ofresistor 340. The other end of resistor 340 is coupled to ground. Theoutput of error amplifier 310 is coupled to a gate of source followertransistor 380.

Pass transistor M_(O) 320 is a p-type metal oxide semiconductortransistor (PMOS). A source of pass transistor M_(O) 320 is configuredto receive Vin, and a drain of pass transistor M_(O) 320 is configuredto produce an output voltage Vout of LDO 300. In some implementations,Vout is at about 0.7 V. The drain of pass transistor M_(O) 320 iscoupled to a source of source follower transistor 380, load capacitor350, and output current sink 360. Load capacitor 350 and output currentsink 360 are coupled to each other in parallel between the drain of passtransistor M_(O) 320 and ground. A gate of pass transistor M_(O) 320 iscoupled to a gate and a drain of noise cancelling transistor M_(D) 370,which are further coupled to a drain of source follower transistor 380.A source of noise cancelling transistor M_(D) 370 is coupled to thesource of pass transistor M_(O) 320 to receive Vin. The drain of sourcefollower transistor 380 is coupled to current sink 390.

In some implementations, error amplifier 310 includes a pair of inputNMOS 311 and 312, a pair of PMOS 313 and 314, a compensation capacitor315, and a bias current source 316. Both sources of NMOS 311 and 312 arecoupled to bias current source 316, which is further coupled to ground.The gates of NMOS 311 and 312 are configured to receive a referencevoltage Vref and a feedback voltage, respectively. The gate of NMOS 312is coupled to feedback resistor 330 and resistor 340 to receive thefeedback voltage from the output of LDO 300 (i.e., the drain of passtransistor M_(O) 320). A drain of NMOS 312 is configured to provide anoutput of error amplifier 310 at the output terminal 310 c, and thedrain of NMOS 312 is coupled to compensation capacitor 315. The pair ofPMOS 313 and 314 are coupled to each other at their sources at voltagesupply input terminal 310 d to receive Vin. The gates of PMOS's 313 and314 are further coupled together to a drain of PMOS 313 and the drain ofNMOS 311. The drain of PMOS 314 is coupled to the output 310 c of erroramplifier 310 with the compensation capacitor 315 and the drain of NMOS312.

During operation, error amplifier 310, and source follower transistorM_(SF) 380 create a slow loop to define the output voltage Vout. Erroramplifier 310 receives Vin at its voltage supply input terminal 310 dand a reference voltage Vref at its positive input terminal 310 a. Theoutput 310 c of error amplifier 310 drives the gate of source followertransistor M_(SF) 380, causing the source of source follower transistorM_(SF) 380 to generate an output DC voltage of Vout. Vout is feedback tothe negative input terminal 310 b of error amplifier 310 via feedbackresistor 330. Thus, when Vout drops, the output voltage of erroramplifier 310 goes up. With the gate-source voltage (Vgs) of sourcefollower transistor M_(SF) 380 staying the same, the source voltage ofsource follower transistor M_(SF) 380 (i.e., Vout of LDO 300) will alsogo up in response. Such a slow loop provides a low resistance at theoutput of LDO 300 (i.e., the node at which the drain of pass transistorM_(O) 320 and the source of source follower transistor M_(SF) 380 arecoupled to), and hence, the transient response of Vout is better thanthe conventional p-type output LDO 200 shown in FIG. 2. Because of thegood transient response, LDO 300 does not require large compensationcapacitor. For example, a compensation capacitor 315 of about 100 fFwould suffice. However, the transient current at the output of LDO 300can be quite large sometimes, such as, for example, rising from 1 mA to10 mA in some implementations. Therefore, a second loop, which is alsoreferred to as a fast loop, is configured within LDO 300 to address thisissue.

In some implementations, a fast loop within LDO 300 is configured toinclude pass transistor M_(O) 320, noise cancelling transistor M_(D)370, and source follower transistor M_(SF) 380. Pass transistor M_(O)320 is biased with a large load current from load current sink 360. Asshown in FIG. 3, the sources of both noise cancelling transistor M_(D)370 and pass transistor M_(O) 320 are coupled to the voltage railsupplying Vin. Noise cancelling transistor M_(D) 370 and pass transistorM_(O) 320 are coupled to each other at their gates. Further, noisecancelling transistor M_(D) 370 is diode connected to create a lowresistance from the supply rail carrying Vin to the gate of passtransistor M_(O) 320. Thus, noise cancelling transistor M_(D) 370 bringssupply noise from Vin to the gate of pass transistor M_(O) 320. As aresult, the supply noise across pass transistor's M_(O) 320 gate-sourcevoltage V_(GS) can be suppressed. The small signal gain from Vin to thegate of noise cancelling transistor M_(D) 370 can function as noisecancelling to improve PSRR. Moreover, the fast loop gives higherbandwidth, which also helps to improve PSRR and transient response.

FIG. 4A shows a loop phase plot and FIG. 4B shows a loop gain plot ofLDO 300 in one study of the slow loop of LDO 300. Referring to FIG. 4A,LDO 300 has a phase margin of about 90 degrees. As shown in FIG. 4B, LDO300 provides about 28 dB DC gain and a gain-bandwidth of about 100 MHz.In sum, the plots in FIGS. 4A-4B demonstrate that the slow loop of LDO300 is relatively stable.

FIG. 4C shows another loop phase plot and FIG. 4D shows another loopgain plot of LDO 300 in one study of the fast loop of LDO 300. Referringto FIG. 4C, LDO 300 has a phase margin of about 77 degrees and again-bandwidth of about 2 GHz. As shown in FIG. 4D, LDO 300 has a loopgain of about 28 dB.

FIG. 4E shows a PSRR plot of one implementation of LDO 300. At lowfrequency, the high gain loop reduces the PSRR. At high frequency, theload capacitor 350 reduces the PSRR. At mid-range frequencies, the fastloop reduces PSR to about −8 dB (see curve 410 in FIG. 4E) according tosimulation, assuming noise cancelling transistor M_(D) 370 is coupled toa clean voltage supply which means there is no noise cancelling. Thefast loop takes advantage of high load current and has highgain-bandwidth. The noise cancelling transistor M_(D) 370, which isdiode-connected, limits the DC gain for the fast loop to create enoughphase margin for the fast loop of LDO 300. As discussed above withreference to FIG. 3, the diode connected noise cancelling transistorM_(D) 370 feeds the supply noise from Vin to the gate of pass transistorM_(O) 320. The supply noise across pass transistor's M_(O) 320gate-source voltage V_(GS) is suppressed. In the example shown in FIG.4E, because the supply noise across pass transistor M_(O) 320 VGS issuppressed, PSR is further reduced to about −14 dB (see curve 420).

FIG. 5 shows a flow diagram of one implementation of a method to providea regulated low voltage from an input voltage (Vin). At box 505, asource of a pass transistor (e.g., M_(O) 320 in FIG. 3) receives aninput voltage Vin. At box 510, the source of the pass transistor iscoupled to a source of a diode-connected noise cancelling transistor(e.g., M_(NC) 370 in FIG. 3). At box 515, a source of a source followertransistor (e.g., M_(SF) 380 in FIG. 3) is coupled to a drain of thepass transistor. At box 520, a drain of the source follower transistoris coupled to a drain and a gate of the noise cancelling transistor. Atbox 525, a current sink (e.g., current sink 390 in FIG. 3) is coupledbetween the drain of the source follower transistor and ground. In someimplementations, the pass transistor, the source follower transistor,and the noise cancelling transistor can be configured into a fast loopof the LDO.

At box 530, an error amplifier (e.g., error amplifier 310 in FIG. 3)receives a reference voltage and a feedback voltage from the drain ofpass transistor. The feedback voltage can be passed to the erroramplifier via a feedback resistor (e.g., feedback resistor 330 in FIG.3). At box 535, the error amplifier generates an output voltage based onthe reference voltage and the feedback voltage. At box 540, the erroramplifier drives the gate of the source follower transistor with itsoutput voltage. In some implementations, the error amplifier, the sourcefollower transistor, and the feedback resistor form a slow loop of theLDO to define the output voltage of the LDO.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples described herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

What is claimed is:
 1. A low dropout (LDO) regulator circuit,comprising: a pass transistor (M_(O)) having a source, a drain, and agate, the source coupled to an input voltage (Vin); a noise cancellingtransistor (M_(D)) having a source, a drain, and a gate, the sourcecoupled to the Vin, the gate coupled to the drain of the noisecancelling transistor and the gate of the pass transistor; a sourcefollower transistor (M_(SF)) having a source, a drain, and a gate, thesource coupled to the drain of the pass transistor, the drain coupled tothe drain and the gate of the noise cancelling transistor; a currentsink coupled between the drain of the source follower transistor andground; an error amplifier having an output, a positive input terminal,and a negative input terminal, the output coupled to the gate of thesource follower transistor, the positive input terminal configured toreceive a reference voltage (Vref); and a feedback resistor coupledbetween the drain of the pass transistor and the negative input terminalof the error amplifier.
 2. The LDO regulator circuit of claim 1, furthercomprising: a load capacitor coupled between the drain of the passtransistor and ground.
 3. The LDO regulator circuit of claim 2, whereinthe load capacitor has a capacitance of 2 pF.
 4. The LDO regulatorcircuit of claim 1, wherein the source follower transistor, the feedbackresistor, and the error amplifier are configured into a first loop. 5.The LDO regulator circuit of claim 4, wherein the pass transistor, thenoise canceling transistor, and the source follower transistor areconfigured into a second loop.
 6. The LDO regulator circuit of claim 5,wherein the first loop is configured to be a slow loop and the secondloop is configured to be a fast loop.
 7. The LDO regulator circuit ofclaim 1, wherein the error amplifier comprises a pair of p-typetransistors configured as a current mirror, and sources of the pair ofp-type transistors are coupled to the Vin.
 8. The LDO regulator circuitof claim 7, wherein the error amplifier further comprises a first n-typetransistor configured as the positive input terminal and a second n-typetransistor configured as the negative input terminal.
 9. A method toprovide a regulated low voltage from an input voltage (Vin), the methodcomprising: receiving the input voltage at a source of a pass transistor(M_(O)); coupling a source of a noise cancelling transistor (M_(D)) tothe source of the pass transistor, wherein the noise cancellingtransistor is configured into a diode; coupling a source of a sourcefollower transistor (M_(SF)) to a drain of the pass transistor, andcoupling a drain of the source follower transistor to a drain and a gateof the noise cancelling transistor; coupling a current sink between thedrain of the source follower transistor and ground; receiving areference voltage (Vref) at a positive input terminal of an erroramplifier; coupling a feedback resistor between the drain of the passtransistor and a negative input terminal of the error amplifier toprovide a feedback voltage to the error amplifier; generating an outputof the error amplifier based on the feedback voltage and the referencevoltage; and driving a gate of the source follower transistor with theoutput of the error amplifier.
 10. The method of claim 9, furthercomprising: coupling a load capacitor between the drain of the passtransistor and the ground.
 11. The method of claim 10, wherein the loadcapacitor has a capacitance of 2 pF.
 12. The method of claim 9, furthercomprising configuring the source follower transistor, the feedbackresistor, and the error amplifier into a first loop.
 13. The method ofclaim 12, further comprising configuring the pass transistor, the noisecanceling transistor, and the source follower transistor into a secondloop.
 14. The method of claim 13, wherein the first loop is a slow loopand the second loop is a fast loop.
 15. The method of claim 9, whereinthe error amplifier comprises a pair of p-type transistors configured asa current mirror, and sources of the pair of p-type transistors arecoupled to Vin.
 16. The method of claim 15, wherein the error amplifierfurther comprises a first n-type transistor configured as the positiveinput terminal and a second n-type transistor configured as the negativeinput terminal.